Today, many combustion engines, especially smaller engines, have an open or ventilated fuel system. This implies that hydrocarbons evaporate from their fuel tanks and emit air pollution even when the engines are not running, but at the same time overpressure in the fuel tank is prevented, which is very important. In most cases, a handheld working tool, such as a chain saw, has a fuel tank with a ventilation filter, usually made of sintered material. Hereby, the tool can be inclined without any significant fuel leakage, but the other problems, such as smell and air pollution, remain.
Through U.S. Pat. No. 1,633,843 a fuel system is known, which comprises a pressure regulator connected to the fuel tank. The regulator prevents fuel and fuel fumes from leaking out of the fuel tank when it is under overpressure. The regulator is mounted in such a way that it protrudes from the fuel tank and is in its entirety located above the fuel level. Due to this location the regulator is heated up by the heat from the adjacent cylinder and cannot significantly be cooled down by the fuel. This implies an increased risk for vapor bubbles being formed in the fuel and thus causing a so called vapor lock in the regulator.